The prior art, of course, is replete with varieties of manual pump dispensers adapted to pump liquid out a discharge orifice from a container on which the dispenser is mounted. One variety of such a dispenser is shown in the McKinney U.S. Pat. No. 4,161,288 in which the piston comprises a tubular inlet stem leading up to an opening in the piston head through which liquid passes into the pumping chamber. The piston seal is in the form of a plastic insert which seals the piston to the cylinder wall. The seal includes a central check plug aligned with the opening by webs integral with the piston seal and plug.
When the piston is raised by the trigger, the central plug seats on the opening blocking exit of the liquid back through the inlet. It raises on the downstroke to permit liquid to flow into the chamber.
Other check valves on the piston have included a steel ball operable in a special chamber and small enough to raise to permit passage of liquid on the downstroke, but dropping to close the tubular stem on the upstroke.